1. Field of the Invention
The present invention relates to the field of telecommunications. More particularly, the present invention relates to a method and a system for eliminating interbeam interference and multipath fading caused by multiple uplink beams coupling in a bent-pipe satellite communications system.
2. Description of the Related Art
In a bent-pipe satellite communications system that combines four spatially adjacent uplink beams, with each beam using spread-spectrum techniques for beam separation, multipath fading can occur if two or more of the adjacent uplink beams are routed on-board a satellite to the same destination earth station receiver, such as an earth station for a terrestrial gateway communications system. While the spread-spectrum code set used with a particular uplink beam would be different from the code set used on adjacent uplink beams, an uplink signal transmitted from the edge of one beam couples equally well to an adjacent beam. If the two adjacent beams carrying the uplink signal are routed to the same gateway receiver, a multipath condition can exist because the same signal has been effectively transmitted on the two adjacent beams and combined prior to being received at the gateway receiver. That is, since RF phasing of the transponders of the two adjacent uplink beams can vary, destructive interference may occur between the two RF paths, resulting in a substantial loss of signal power at the gateway receiver. In effect, the gateway receiver receives the same signal from two different sources by the intercoupling of uplink signals between adjacent uplink beams. In addition, second tier interference may be introduced by transmitters in other beams utilizing the same code sets.
FIG. 1 shows an exemplary multi-beam bent-pipe satellite communications system 10 that illustrates multipath fading caused by an uplink beam signal being coupled into two adjacent uplink beams and second tier interference. Satellite communications system 10 uses four spread-spectrum code sets, based, for example, on synchronous Walsh codes, for separating uplink beams 12, 13, 17 and 18 in a well-known manner, each of which accept transmitted signals using uplink frequency F1. The exemplary code sets are referred to herein as W1-W4. A ground station 11 transmits an uplink communications signal using uplink beam 12. While only ground station 11 is shown within the geographic region covered by beam 12, there are a plurality of ground stations within the geographic region covered by beam 12, but are not shown. Ground station 11 is geographically located near the edge of beam 12 such that the transmitted signal is also coupled into adjacent uplink beam 13. Beams 12 and 13 are received by satellite 14, combined with each other (and beams 17 and 18), and transmitted over a downlink beam 15 to a common earth station receiver 16, such as a terrestrial gateway. System 10 includes a plurality of gateway receiver stations, of which only gateway receiver 16 is shown. Multipath fading occurs at gateway receiver 16 because multiple versions of the signal from ground station 11 are received (effectively) from two different paths and are potentially destructively combined prior to being received by gateway receiver 16. Other beams, for instance, beam 19, which utilizes the same code set as beam 12 (and potentially the same code as ground station 11), couple into the beams adjacent to beam 12, such as beams 13 and 18, causing second tier interference when combined by the satellite and received by gateway receiver 16.
FIGS. 2 and 3 respectively show conventional uplink beam tiling patterns for a reuse of four which can be used in satellite communications systems. Each hexagon shown in FIGS. 2 and 3 represents a different geographic region covered by a corresponding uplink beam. The numbers within the hexagons represent a partitioning of resources (such as frequencies, polarities, or code sets) between groups of geographic regions. All the users in geographic regions labeled 1 use the same resources, which are different from those used in regions 2, 3 and 4. A similar statement can be made for each regions 2, 3, and 4. The patterns shown in FIG. 2 and 3 are representative, and other tiling patterns for sharing resources between groups of regions 3, 7, 8, etc., also exist. Also shown in an outlined area of these Figures is a grouping of hexagons referred to herein as a "Beam Group". Each number of a beam group is assigned a different set of resources. By replicating a beam group multiple times, a full tiling pattern can be generated. It is assumed for this example, that each beam group is serviced by a different gateway receiver, although this is not required by the present invention. For example, see application Ser. No. 08/953,020.
FIG. 4 shows the tiling pattern of FIG. 2 applied to a conventional Frequency Division Multiple Access (FDMA) based system utilizing four different uplink frequency groups F1, F2, F3 and F4. Again, the outlined section indicates a beam group. FIG. 5 shows the tiling pattern of FIG. 2 applied to an FDMA-based system utilizing different combinations of uplink frequency groups F1 and F2 and two polarizations P1 and P2. FIG. 6 shows a conventional uplink beam frequency tiling pattern for a Code Division Multiple Access (CDMA) based system. Isolation between users in different beams for such a system is provided by proper allocation of code sets between the beams. FIG. 7 shows the tiling pattern of FIG. 2 applied to codes for a representative CDMA-based system utilizing code sets W1-W4 for isolation between the beams.
Each of the tiling patterns of FIGS. 2-7 suffer from multipath fading that occurs when two adjacent uplink beams are downlink-routed to the same terrestrial gateway receiver. Also, they suffer from second tier interference through the introduction of signals that are twice removed from a desired beam, using the same resources, and are coupled via adjacent beams. In more advanced tiling schemes (such as described in application Ser. No. 08/953,020 and shown in FIG. 8) this "second tier" interference may still exist, although not strictly originating from second tier beams. In an FDMA system, this multipath condition and interference is easily remedied through the application of appropriate filtering on-board the satellite. However, for CDMA systems where adjacent beams use the same frequency and polarization, the filtering techniques that are applied in an FDMA system cannot be used. Consequently, what is needed is a method and a system that eliminates the multipath fading and interference effects caused by an uplink transmission being coupled into adjacent uplink beams utilizing the same frequency and polarization, and downlink-routed to the same terrestrial gateway receiver, or routed to other terrestrial gateway receivers with other signals using the same code set.